A commonly used detector of this type is shown in FIG. 1. It comprises a spectrometer 1, a secondary pump 2, and a primary pump 3 whose suction inlet is connected to the delivery from the secondary pump 2 outlet via a pipe 4 fitted with a valve 5. The inlet 6 of the detector is connected by a pipe 7 fitted with a valve 8 to the suction inlet of the primary pump 3. A pipe 9 fitted with a valve 10 for so-called "direct" measurement connects the pipe 7 to the spectrometer 1.
Such a detector is very well suited to detecting small leaks in seals in test parts, however very large leaks cannot always be detected or it can take a very long time to detect them.
After the spectrometer 1 has been evacuated, the part under test, connected to the inlet 6, is pre-evacuated by the primary pump 3, the valves 5 and 10 are closed and the valve 8 is open, the pump 2 is kept running, and the spectrometer 1 is in operation. The valve 5 can be opened for performing measurements in counterflow only after the inlet pressure to the detector has dropped at least down to 10 mbar, and often down to 1 mbar. This pressure must be compatible with the maximum outlet pressure of the secondary pump and must enable the gas analyzer to operate properly. If the leak is large, the time required to reach this pressure level can be very long; and indeed it might never be reached if the leak is very large. Under such circumstances, it can nevertheless be deduced that there is a large leak because the pressure does not go down. A more serious situation relates to detecting leaks on sealed parts. Under such circumstances, the parts to be inspected are sealed under a tracer gas, or else they are sealed under air or a dry gas other than the tracer gas and are then pressurized in an enclosure containing the tracer gas at a pressure of the order of a few bars. If such a part has a leak, then helium penetrates into the part through the leak. After being immersed for several hours, the part is removed from the pressurized helium enclosure and is placed in an enclosure which is connected to the inlet 6 of the detector. If a part containing a tracer gas (helium) has only a small leak, then pre-evacuation of the enclosure by the primary pump 3 makes it possible to achieve a pressure at the inlet to the detector which is compatible with operation of the mass spectrometer 1 and thus to open the valve 5 long before the part has lost all its helium. It is therefore very easy to measure the leak; however, if on the contrary the leak is very large, then all the helium contained in the part can be evacuated therefrom and from the enclosure solely during the pre-evacuation stage, and thus by the time the pressure has reached a value that enables the valve 5 to be opened, all or most of the helium may already have been removed, in which case measurement will indicate no leak or only a small leak.